The present invention generally relates to treatment fluids used for stimulation and remediation operations in subterranean formations, and, more particularly, to treatment fluids that contain a boron trifluoride complex and methods for using such treatment fluids.
Treatment fluids can be used in a variety of subterranean operations. Such subterranean operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments and the like. As used herein, the terms “treat,” “treatment,” and “treating” refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid. Illustrative treatment operations can include, for example, fracturing operations, gravel packing operations, acidizing treatments, scale dissolution and removal, consolidation treatments, and the like. In alternative embodiments, treatment operations can refer to an operation conducted in a pipe, tubing, or like vessel in conjunction with achieving a desired function and/or for a desired purpose (e.g., scale removal).
In acidizing treatments, for example, subterranean formations comprising acid-soluble components, such as carbonate and sandstone formations, can be contacted with a treatment fluid comprising an acid to dissolve the formation matrix. After the acidizing treatment is completed, the treatment fluid and salts dissolved therein can be recovered by producing them to the surface (e.g., “flowing back” the well), leaving a desirable amount of voids or conductive pathways (e.g., wormholes in carbonates) within the formation. Acidizing operations can enhance the formation's permeability and can increase the rate at which hydrocarbons are subsequently produced from the formation.
Acidizing a siliceous formation (e.g., a sandstone formation, a clay-containing formation or a like aluminosilicate-containing formation) can introduce certain challenges that are not present when acidizing a carbonate formation. As used herein, the term “siliceous” refers to the characteristic of having silica and/or a silicate, including aluminosilicates. Most sandstone formations are composed of about 40% to about 98% sand quartz particles, i.e., silica (SiO2), bonded together by varying amounts of a cementing material including carbonate (calcite or CaCO3), aluminosilicates, and silicates. Carbonate formations can usually be effectively treated with a variety of acid systems, including mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids), often with similar success, where the acidity of the treatment fluid alone can be sufficient to solubilize formation cations. The treatment of siliceous formations with these acids, however, can have little or no effect because most organic and mineral acids do not react appreciably with the siliceous minerals characterizing these formations.
By far the most common method of treating sandstone and other siliceous formations involves introducing corrosive, very low pH fluids comprising hydrofluoric acid into the well bore and allowing the acid to react with the formation matrix. In contrast to other mineral acids, hydrofluoric acid can be very reactive with aluminosilicates and silicates (e.g., sandstone, clays and feldspars). In some cases, hydrochloric acid can be used in addition to hydrofluoric acid in the treatment fluid to maintain a low pH as hydrofluoric acid becomes spent during a treatment operation, thereby retaining certain dissolved species in a highly acidic solution. Hydrofluoric acid acidizing can often be used to remove damage that is present within the subterranean formation.
Although treatment fluids containing hydrofluoric acid and, optionally, another acid can be desirably used to affect dissolution of siliceous minerals, the use of low pH fluids can have detrimental consequences in certain instances. Specifically, at low pH values, dissolved fluoride ions can precipitate and damage the subterranean formation, particularly in the presence of certain cations such as, for example, Al3+, Group 1 metal ions (e.g., Na+ and K+) and/or Group 2 metal ions (e.g., Mg2+, Ca2+, and Ba2+). In some cases, this precipitation can damage the formation more than if the original treatment operation had not been performed at all. For instance, hydrofluoric acid tends to react very quickly with authigenic clays (e.g., smectite, kaolinite, illite and chlorite), especially at temperatures above 200° F. and below pH 1, as a function of mineral surface area. Because of this rapid reaction, the hydrofluoric acid can penetrate only a short distance into the formation before becoming spent. Simultaneously, precipitation of various aluminum and silicon compounds can occur as increasing amounts of siliceous minerals are dissolved at low pH. At certain temperatures and subterranean conditions, dissolution of a sandstone matrix or like siliceous material can sometimes occur so rapidly that uncontrolled precipitation can become an inevitable problem. The precipitated products can plug pore spaces and reduce the porosity and permeability of the formation, thus impairing flow potential. In addition, low pH treatment fluids containing one or more acids can present corrosion and safety issues.
The precipitation of calcium fluoride, fluorosilicates, and other insoluble fluoro compounds during hydrofluoric acid treatments can be of particular concern, since production can be delayed while damage control operations are conducted. Fluorosilicates can be especially problematic because they are the primary product of the dissolution of a clay and hydrofluoric acid. In addition, fluorosilicates can be difficult to remediate through redissolution. Calcium fluoride can be a later concern in the process, because the fluoride anion needs to be present in its free ion form, and that does not happen until a higher pH is reached after some of the acid becomes spent. Unlike fluorosilicates, calcium fluoride can be remediated, in some instances. Remediation techniques can include a commercially available treatment system from Halliburton Energy Services, Inc. known as “F-SOL” acid system, which can be used to dissolve calcium fluoride. Fluoroaluminate formation can also be of a significant concern due to the reaction of fluorosilicates with clay minerals. Fluoroaluminates are thought to be soluble as long as the pH is below about 2 and the ratio of F/Al is maintained below about 2.5. If precipitated, their dissolution typically requires strong HCl (>5%).
Avoiding the formation of calcium fluoride, fluorosilicates, or other insoluble fluoro compounds can be a primary design objective in a treatment operation conducted in a subterranean formation or elsewhere. Various means have been used with mixed success to accomplish the foregoing. Blends of organic acids and hydrofluoric acid can be used to slow the dissolution kinetics of sandstone formation solids. However, since organic acids typically have higher pKa values than do mineral acids, precipitation can become problematic as the treatment fluid's pH rises. Pre-flush sequences with mineral acids can be used to remove calcium salts from sandstone formations, before the main acidizing sequence is conducted to remove formation aluminosilicates. Generally, these flushes do not spend completely and typically return, upon flowback, with a persisting low pH. In addition to presenting safety issues, the return of an acidic fluid can result in corrosion of downhole tubular goods (including coiled tubing) and surface equipment. Other multi-stage sandstone acidizing treatment operations can also be used, particularly to remove calcium ions.
Chelating agents can also be included in treatment fluids to sequester at least a portion of the formation cations that cause unwanted precipitation. Likewise, chelating agents can be used in treating pipelines, tubing, and like vessels by removing metal ion scale from the pipeline or tubing surface. However, there are certain operational concerns that can be encountered with the use of many common chelating agents. First, many common chelating agents are not biodegradable or raise toxicity issues that can make their use in a subterranean formation problematic. Further, the available salt form of some chelating agents can actually exacerbate precipitation problems in a treatment operation rather than lessening precipitation.
Although chelating agents can extend the conditions under which treatment fluids containing hydrofluoric acid can be effectively used, even better precipitation control over a wider range of pH values, while still achieving a satisfactory dissolution rate of siliceous materials, would be highly desirable from an operational standpoint. Furthermore, in terms of safety and ease of handling, it would also be desirable to be able to affect dissolution of siliceous materials without resorting to the use of highly acidic treatment fluids.